Radio communication systems utilizing multiplexed noise codes are generally known. A typical example is shown and described in U.S. Pat. No. 4,293,953, entitled, "Bi-Orthogonal PCM Communication System Employing Multiplexed Noise Codes", which issued to Frank S. Gutleber, the present inventor, on Oct. 6, 1981.
The concept of code expansion and compression for the general class of multiplexed noise codes comprised of noise code mate pairs having autocorrelation functions which upon detection in a matched filter provide an impulse is also generally known. Such concepts are disclosed, for example, in U.S. Pat. No. 3,461,451, entitled, "Code Generator To Produce Permutations Of Code Mates", which issued to the present inventor on Aug. 12, 1969; U.S. Pat. No. 3,519,746, entitled, "Means And Method To Obtain An Impulse Autocorrelation Function", which issued to the present inventor on July 7, 1970; and U.S. Pat. No. 3,634,765, entitled, "System To Provide An Impulse Autocorrelation Function . . . ", which issued to the present inventor on Jan. 11, 1972.
In the above mentioned related abandoned application, U.S. Ser. No. 502,416, entitled, "Code Compressor For Interleaved Multiplexed Noise Codes", there is disclosed the concept of repetitively compressing corresponding code mate pairs in successive stages to provide ever diminishing codes of half the bit quantity and twice the amplitude until a single pulse results at the location of the final code bit whose amplitude is increased by a factor of twice the number of bits in each of the original interleaved codes. The apparatus employed comprises a series of cascaded code separators separated by delay lines providing ever increasing delays which are multiples of the pulsewidth of each code bit and wherein the delay is doubled at each stage until the last stage is reached and a delay of one half the original code length of the interleaved codes is provided, whereupon a combining of the codes results in an impulse autocorrelation function.
In the aforementioned related application, U.S. Ser. No. 536,064, entitled, "Expanded Multilevel Noise Code Generator Employing Butting", there is disclosed the concept of the generation of code mate pairs comprising a pair of expanded noise code mates having code portions of more than one amplitude level and which are generated by butting noise code mates of different amplitude levels such that mutually transposed butted code mates in the expanded code mate pairs have respective code bits which are amplified by a predetermined gain factor and wherein one of the butted code mates comprises the complement or negative of the original code mates.
One known method for compressing a code mate pair is by employing a matched filter comprised of a separate tapped delay line for each code with the output of the taps matched to the input code bits in reverse order to the input sequence. The linear summation of the matched outputs in each delay line then provides the compressed code for each code of the pair. Each output, moreover, is equal to the autocorrelation function of the respective code being matched and the simple linear sum of the two outputs results in a single lobeless output signal. This approach, however, becomes relatively expensive and difficult to implement for extremely long codes since a separate tap and amplifier with or without an inverter is required for each of the input code bits.
Accordingly, it is an object of the present invention to provide an improvement in the compression of noise codes.
Another object of the present invention is to provide an improvement in the compression of multiplexed code mate pairs having more than one amplitude level.
Still another object of the invention is to provide an improvement in the compression of multilevel, multiplexed code mate pairs generated by the process of butting code mates.